Dang, that's a toughy. Almost every meeting of my Anime Club I get someone addicted to the Dresden books, but the RPG setting.... I'll give it a shot or 2.
When pitching you have to keep it short, but you can't simplfy Dresden too much. You have to convey the following information about the Dresdenverse:
1. The supernatural world is real and every piece of fantasy is open game
2. Except for the magic and stuff, it is still the real world. The supernatural is in addition to the normal world.
3. Your role in the game is to protect humanity from very nasty things
4. The kind of characters the setting allows are people like Harry Dresden, a Wizard in the Phone book, under Wizards
1st Attempt aka the Don LaFontaine Approach:
There's another world out there, a world of things that go bump. The classic fantasy nasties are real and the Normal world is one big smorgasbord. The only people who can protect humanity and keep this world underwraps are the ones with power or who are smart enough and clever enough to survive or are just too stupid to be scared. People like Harry Dresden, the only man in the Yellow Pages under Wizards.
2nd Attempt:
Vampires, Werewolves, Shakespearean Faires and Magic are the tip of the iceburg...of what's around your neighborhood. A supernatural world exists out there and keeping it under wraps are people like the White Council of Wizards. One such member is Harry Dresden, a Wizard who consults for the Chicago police, everyone wants a piece of and advertises in the Phonebook, under Wizards.